By necessity and design, semiconductor integrated devices have grown into increasingly complex, dense circuits on silicon with extremely fine interconnecting metal lines and ultra-thin dielectric gates. In consequence, very large-scale integrated (VLSI) devices are more sensitive to the thermal processes associated with their last manufacturing stages, and in particular with the final step, sealing each silicon chip in its individual ceramic housing. The increased size and pin count of VLSI devices dictate larger two-dimensional hermetic packages.
Semiconductor ceramic packaging is rapidly evolving into flat geometries such as chip carriers, pin grid arrays, and other surface-mounted packages. These packages are designed with a gold-plated metal lid made of Kovar (same expansion as alumina) bonded with a low-temperature gold solder to a gold-plated metal ring on the surface of the ceramic base. Ceramic packaging represents a major cost factor in the manufacture of integrated circuit devices, much of the cost going into gold usage. Since a primary driving force in the semiconductor industry is continual price reduction, it is clear that cost-reducing material technology will progressively gain importance.
Sealing with gold provides numerous technical advantages. These include a sealing temperature in just the right range to guarantee optimum device reliability (350.degree. C.), excellent seal strength and retained hermeticity under extensive thermal shocks as well as very low moisture entrapment in the inner cavity surrounding the silicon chip.
Lower packaging material cost can be achieved through the use of lead borate sealing glasses. These glasses remain a viable alternative to gold alloy as sealing agents. It is well known in the art that elemental gases and moisture diffusion rates through lead borate and lead borosilicate glasses are extremely low, even lower than through fused silica, which itself is permeable to helium.
Relatively higher seal temperature, however, limits the use of solder glasses since no commercial sealing glass will seal below 420.degree. C. Sealing glasses can at present be used only with semiconductor devices that can tolerate 420 C. or higher. This temperature restriction precludes sealing with glass the newer generation of VLSI and other temperature-sensitive semiconductor circuitry (Group III-V compounds).
To date serious attempts to design a practical and reliable lower temperature (in the range of 300-400.degree. C.) sealing glass have met with formidable technical barriers. The search is hampered by the fact that in the course of new material evolution the design of glasses remains a largely empirical science.
The specifications for a semiconductor ceramicpackage sealing glass are numerous and demanding. Somehow these must be met with one single chemical formulation preferably produced as a glass melt rapidly quenched to room temperature. The basic material and processing requirements for a commercially practical sealing glass can be listed as:
1. a metal oxide mixture in the form of a true solution (homogeneous melt),
2. glass formation during rapid cooling of the melt (solidified liquid),
3. low glass viscosity at seal temperature (350.degree. C.),
4. no tendency to crystallize (glass stability) during seal formation and completion,
5. a reasonably low linear thermal expansion (50 to 110.times.10.sup.-7 /.degree.C.),
6. ease of linear thermal expansion adjustment by the addition of a lower expansion ceramic filler,
7. glass chemical stability (insoluble in water, resistant to acids, alkalies and hot water),
8. good wetting and high bonding strength to alumina ceramic surfaces,
9. no presence in the formula of alkali or other fast-migrating ions (electronic applications) or volatile components that create serious health hazards (such as arsenic oxide, thallium oxide, etc.),
10. capacity of producing a strong, tight and hermetic seal to a glass, metal and ceramic surface and capability of surviving several hundred cycles of thermal shocks, liquid to liquid, condition C (MIL-STD-883), and
11. ease of commercial processing.
In contrast to the lead borate glass system, the lead vanadium oxide binary is another low-temperature glass-forming system of potential interest since the glass corresponding to the PbO--V.sub.2 O.sub.5 eutectic (one mole PbO to one mole V.sub.2 O.sub.5) has a soften point at about 250.degree. C., notably lower than the most fluid glass in the lead boron oxide binary (12% B.sub.2O.sub.3 - 88% PbO).
However, severe obstacles are associated with lead vanadium glasses. Foremost is the problem of thermal instability, particularly when the glass is reduced to a powder (the glass powder will rapidly recrystallize on reheating). Second, the marked tendency for vanadium oxide to lose oxygen and change from the pentavalent (V.sub.2 O.sub.5) state to the trivalent (V.sub.2 O.sub.3) state, a refractory su Further, there is the pronounced chemical reactivity of V.sub.2O.sub.5 with organic liquids.